1. Field of the Invention
The present invention relates to a novel and improved coating composition and process for applying a styrenic based adhesive, cement, coating, or paint to thermoplastic polymer surfaces, particularly to toy articles formed of elastomeric materials. Because the coating composition uses naturally occurring terpene solvents and other high evaporation rate organic solvents, it is safe for use as a coating or paint for toy articles which can be safely used by children.
2. Description of the Related Art
Children's toys and dolls, in particular, have a history dating back into antiquity. Generally, the technology surrounding the manufacture of dolls centers on creation of an attractive doll face, miniturized application of conventional clothes, manufacture of a doll body using plush or other sculptural techniques, and the simulation of the hair with sculptural elements, fibers, or other techniques. In the case of doll faces, the doll maker has a wide panoply of possible constructions and manufacturing techniques at his disposal. Traditional techniques involve the use of plaster-like or hard plastics material to cast a doll face. In time, however, soft rubber materials such as styrene-butadiene rubber (SBR) and styrene-butadiene-styrene block copolymer (Kraton) elastomers have come to replace plaster and hard plastics in the production of doll heads because of their realistic look and feel.
The soft rubber doll faces include all the normal features of a human figure simulated by the doll including the entire head, including well formed lips, teeth, cheeks, nose, chin, ears, and forehead. After the injection molding of an elastomeric doll face, it is finished by applying a dye or other similar material to give the face a natural skin color. Parts of the facial skin are often given a contrasting reddish color to improve the attractiveness of the face, as by using an air brush may be used to apply a patch of rosy hue to the cheeks.
Regardless of the type of paint used on a toy doll article, it has remained imperative that such materials be safe for young children. Safety requirements have evolved through the years as safety concerns have grown. Generally, safety requirements mandate that any material compositions used in a toy be odorless, nonirritating to the skin or eyes or the like, and be nontoxic if ingested. Additional requirements have been expected of materials used to coat or paint toy articles in that they must be non-peeling, requiring that the coating or paint tenaciously adhere to the toy item to avoid flaking or peeling and possible consumption by a child user. Accordingly, such paints should be resistant to oxidation and flaking, particularly as applied to elastomeric rubber surfaces of soft doll faces. And further the paint should be able to withstand the stretching and other physical abuse of a child so that the appearance of the painted surface is maintained. And above all the paint or coating composition must avoid the use of undesirable volatile organic solvents the residual presence of which can be dangerous due to the toxic effect of certain aromatic and chlorinated hydrocarbon solvents.
In view of the environmental, health and safety concerns in the use of highly toxic organic solvents, less controversial solvents such as water or mineral spirits have been used to provide toy paint or coating compositions. The mineral spirits solvent is less toxic and considered generally safe in coating applications for toys and, of course, water is completely safe and non odiferous as a coating solvent for any coating applications. However both solvents evaporate relatively slowly over a long period of time which detracts from the overall effectiveness of these coating compositions. For example, when applying a water based latex emulsion paint to a soft elastomeric item, a slower evaporation time delays set up of the paint coating composition and inhibits complete bonding between the joined surfaces of the applied coat and the article. As a result of an extended drying period, non-uniform and unstable coatings result leaving these coating vulnerable to use factors which generate peeling and splitting among other negative consequnces.
As indicated above, the class of the styrenic elastomeric materials commonly used to mold doll faces and other toy articles are A-B-A type block polymers such as styrene-butadiene-butylene copolymer-styrene or styrene-butadiene-styrene, manufactured by Shell and sold under the trademark Kraton. The molded polymeric doll faces tend to be dull and unattractive and so are finished by application of a skin coating or paint to provide a good facial appearance to this facial piece. It has now been found that an elastomeric adhesive coating comprising a styrene based resin and a terpene solvent has particular advantages in forming tenaciously bonded adhesive coatings on styrenic resin molded items.
In copending commonly assigned U.S. patent application Ser. No. 09/729,071 (filed Dec. 4, 2000) to Sneddon there is provided a novel paint and adhesive coating composition comprising a styrenic copolymer and a terpene solvent. As described, this coating is particularly effective with styrenic containing elastomeric toy surfaces because of the solubilizing effect of the terpene on both the styrenic copolymer partner in the coating composition and the elastomeric styrenic substrate surface. Additionally, the terpene solvent is both quick drying and safe for usage as a coating ingredient for toy items.
While the styrenic copolymer/terpene solvent composition of the copending application is generally effective, certain challenges arise from its use. For example, in manufacturing environments the terpene solvents, while non-toxic, are odiferous (albeit pleasantly), and use of lesser amounts in coating compositions is desirable in confined manufacturing facilities. Additionally, the efficient through-put of toy manufacturing requires a high evaporation rate of any solvent used in the coating compositions to effect quick drying of a paint or adhesive coating on a substrate. In the case of terpenes, their evaporation rates are adequate but need improvement for greater throughput in the manufacture of painted toy articles. It has been further found that a terpene solvent such as d-limonene has an aggressive affinity for elatostomeric styrenic substrates which can be destructive to a skin substrate when a high concentration of terpene is employed in the coating. These concerns are met by the instant invention which provides for the use of certain co-solvents for the effective terpenes to form paint or adhesive compositions for toy articles which are safe, efficient, and effective when applied to elastomeric styrenic toy substrate surfaces.